The invention relates to electrical keyboards, and more particularly to apparatus for selective backlighting individual keys of electrical keyboards.
A wide variety of keyboards are utilized for data entry terminals and remote control terminals. Some state-of-the-art keyboards are quite complex, as the keys often are designed to provide tactile feedback to the user to assure him or her that electrical contact has been made by depression of the key. In many cases, it is desirable to have a particular selected key illuminated at certain times, for example, to indicate when the key has been properly depressed, or to indicate the status of a function controllable by the key.
Perhaps the state-of-the-art in backlit keyboards is best indicated in U.S. Pat. Nos. 4,288,672, 4,343,975 and 4,489,227. U.S. Pat. No. 4,489,227 shows a horizontal substrate with a plurality of vertical light pipes that each extend upward from an integral horizontal substrate into a transparent key. A common light source feeds light from an external source into the substrate, which guides the light into all of the light pipes, thereby simultaneously illuminating all of the keys. A foam overtravel pad rests on the horizontal support plate. The overtravel pad includes apertures that admit the light pipes. A membrane switch supported by the overtravel pad also includes apertures that allow the light pipes to pass through. U.S. Pat. No. 4,288,672 shows a structure in which a common light source produces 1ight that is reflected by sloped surfaces on the bottoms of light pipes that extend upward through a circuit board assembly. U.S. Pat. No. 4,343,975 discloses a system in which light from a common remote source is routed through an optical conductor to various transparent keys.
The backlit keyboard systems disclosed in the foregoing patents do not allow selective backlighting of individual keys, nor do the patents show how practical, economical manufacture of a keyboard with selectively backlit keys can be accomplished.
Yet, another prior structure provides individual light emitting diodes that are mounted on the bottom surface of a printed circuit board and extend upward through openings in the printed circuit board into very shallow, transluscent 5 mil thick key caps to backlight the key caps. That technique is practical only if the key cap material is very thin. However, most key caps that are molded integrally from rubber-like sheets must be so thick that they cannot transmit enough light from light emitting diodes mounted in this manner.
Those skilled in the art know that in order to accomplish economical manufacture of a printed circuit board using conventional wave soldering techniques, all of the components, including light emitting diodes, must be mounted on one side of the printed circuit board. At the present state-of-the-art, any practical technique for providing backlit keys should provide all components on the "bottom" surface of the printed circuit board to be able to use the conventional wave soldering techniques.
Thus, there remains an unfulfilled need for an economically manufacturable keyboard with selectively backlit keys.